The purpose is to develop and initially characterize a novel transgenic mouse model in which the circulating levels of GH are normal but can be titrated downwards using a tetracycline-repressible transactivators system. Aim 1 is to develop a knockout-knockin transgenic mouse model which will [1] knockout the endogenous murine GH gene and [2] insert a DNA construct which will place the expression of the tetracycline (Tet)- repressible transactivator protein (tTAr) under the control of the endogenous mouse GH promoter. The transactivator will in turn activated a minimal transactivator-dependent (TD) promoter to drive the expression of the rat GH gene (rGH). With this strategy the endogenous GH promoter will essentially drive the expression of rat GH. Since all of the normal regulatory mechanisms will be intact, GH levels should approximately the normal levels found in the mouse and should show the pulsatile release characteristic of this hormone. Aim 2 will characterize and validate the model by first determining how well the knockout- knockin model recapitulates the normal secretion of GH. Plasma GH and insulin-like growth factor (IGF-1) will be measured in homozygous knockout-knockin ve3rsus wild-type mice. The rate of growth will also be assessed in these two groups of animals. Aim 2 will also determine if progressively higher dosages of Tet can be used to titrate the circulating levels of GH and IGF-1 in knockout-knockin mice. Initially, Tet will be administered in the drinking water. Should this approach prove to not be optimal, then subsequent studies will assess the ability of subcutaneously administered slow release tablets of Tet to titrate GH. The knockout- knockin mouse will e used in an ensuing R01 application to test the hypothesis that [1] reduced GH levels early in life may be advantageous to enhancing life span while [2] a moderate increase in GH levels in later life may also contribute to longevity.